So I think something is up with my notifications. Today is the 25th and I just now got the system notification for the coloring contest. The one that was according to the notification posted on the 19th.
I also had this happen with a forum ping earlier this week, where I got notification for a ping that was sent to me on the 2nd. I unfortunately can't verify how long the delay on that one is, I just remember being weirded out that I had to scroll way far back in a thread to even find the ping since it was so late.
Not sure what other information to add here besides that this seems to have started happening this month and that I'm on firefox.